Radioactive materials are often detected and identified by measuring gamma-rays and/or neutrons emitted from the materials. The energy of gamma-rays is specific to that particular material and acts as a “finger print” to identify the material. Similarly, neutron emissions may be used to identify the material. Of high value are detectors capable of identifying the distinctive time-correlated signatures corresponding to neutrons and gamma rays, or “gammas” emitted by fissioning material from within a background of uncorrelated natural radiation. A detector capable of distinguishing neutrons from gammas, as well as offering a fast response time typically has better capability for detecting the distinctive time-correlated events indicative of the presence of fissioning nuclei.
The ability to detect gamma rays and/or neutrons is a vital tool for many areas of research. For example, gamma-ray/neutron detectors allow scientists to study celestial phenomena and diagnose medical diseases. Neutron imaging is used in non-destructive evaluation of complex objects. Additionally, gamma and neutron detectors are important tools for homeland security, helping the nation confront new security challenges. The nuclear non-proliferation mission requires detectors capable of identifying diversion or smuggling of nuclear materials. Government agencies need detectors for scenarios in which a terrorist might use radioactive materials to fashion a destructive device targeted against civilians, structures, or national events. To better detect and prevent nuclear incidents, the Department of Energy (DOE) and the Department of Homeland Security (DHS) are funding projects to develop a suite of detection systems that can search for radioactive sources in different environments.
One particularly useful type of radiation detection, pulse shape discrimination (PSD) provides means for high-energy neutron detection in the presence of gamma radiation background by utilizing the difference in the shapes of scintillation pulses excited by neutrons (recoil protons) and gamma (γ)-rays in organic scintillators. PSD phenomena in organic scintillators are based on the existence of multiple components in the fluorescent decay. In addition to the principal prompt component decaying exponentially (prompt fluorescence), slower emission components with longer decay times (delayed emission) exhibit increased amplitudes for neutron scintillation events, compared to gamma scintillation events. According to a commonly accepted mechanism shown in FIG. 1, the fast component results from the direct radiative de-excitation of excited singlet states (S1), while the slow component originates from the collisional interaction of pairs of molecules (or excitons) in the lowest excited τ-triplet states (T1).
Since the triplet is known to be mobile in some compounds, the energy migrates until the collision of two triplets collide and experience a process, shown as Equation 1:T1+T1→S0+S1  Equation 1
In Equation 1, T1 is a triplet, S0 is the ground state, and S1 is a first excited state. Finally, the delayed singlet emission occurs with a decay rate characteristic of the migration rate and concentration of the triplet population, and is represented by Equation 2:S1→S0+hv  Equation 2
In Equation 2, hv is fluorescence, while S0 is the ground state and S1 is a first excited state. The lifetime of the delayed emission is determined by the lifetime of T1 and the rate of T1T1 collisions. The short range of the energetic protons produced from neutron collisions yields a high concentration of triplets, compared to the longer range of the electrons from the gamma interactions, leading to the enhanced level of delayed emission with longer decay times in neutron-induced pulses in comparison to those produced by the gamma excitation. The observation of PSD in organics with phenyl groups is believed to be, in part, related to the aromatic ring structure, allowing for the migration of triplet energy.
FIG. 2A shows a plot of average waveforms for a stilbene test crystal indicating different levels of delayed light in neutron and gamma scintillation pulses. As can be seen from the plot, some light is produced by the crystal almost immediately, referred to as prompt light, and other light is produced by the crystal over a period of time, referred to as delayed light. Generally, the plot for each type of radiation will have a steep component 202 and a tail component 204. The upper line in the plot represents neutron light decay, while the lower line represents gamma (γ) light decay. As shown in FIG. 2A, the shape for the neutron response has a large tail component 204, which is much smaller or almost negligible for gammas. Thus, stilbene is able to differentiate between the neutron and gamma light decays, and produces noticeably different lines for each radiation type. Unfortunately, growth of stilbene single crystals is difficult and costly, and the crystals are fragile and extremely sensitive to thermal and mechanical shock, making them difficult to field and creating demand for plastic-based alternatives.
Modern waveform digitizers allow for easy separation of neutron and gamma pulses, enabling instrumentation of PSD scintillators into fieldable detectors, as shown in FIG. 2B. The waveforms may be numerically integrated over two time intervals: ΔTotal and a subinterval ΔTail (e.g., for >50 ns), corresponding to the total charge and the delayed component of the signal, respectively. The value of the ratio of charge R=QTail/QTotal for the two time intervals indicates whether the considered event was likely produced by a neutron (high R value) or a gamma ray (small R value). The plot shown in FIG. 2B reveals the presence of both neutrons (upper scatter points) and gammas (lower scatter points) in a plot of the ratio of charge (QTail/QTotal) versus the pulse height.
FIG. 2C illustrates one approach where the neutron/gamma delayed light separation, S, in the stilbene test crystal is used for calculation of the PSD figure of merit (FOM). The PSD separation, S, refers to the gap between the mean ratio of charge (QTail/QTotal) for gamma rays and the mean ratio of charge (QTail/QTotal) for neutrons taken over an extended period of time. The larger the separation, S, the better the organic crystal is at PSD for distinguishing gammas and neutrons.
The PSD technique is most frequently utilized for discrimination between fast neutrons (recoil protons) and gamma-rays (Compton electrons) using liquid scintillators and a few organic crystals. Recent developments broadened the group of PSD materials to include scintillating plastics.
Plastic scintillators are radiation detectors with multiple fields of use that may be easily and inexpensively manufactured in different configurations and sizes. However, being comprised of materials with a low effective atomic number (Zeff), hydrocarbon-based plastics exhibit inefficient photoelectric absorption and are thus rarely used to detect gamma radiation. The photoelectric cross section scales as Zeff4. Accordingly, in one approach, a high Z component may be added to a plastic scintillator to increase photoelectric absorption and sensitivity to gamma radiation. One disadvantage with this approach is the reduction in light yield with increasing concentration of the high Z component. This reduction in light yield associated with high Z loaded plastic scintillators is thought to arise from the conversion of singlet excitons to triplet excitons via spin-orbit coupling to the high Z component. The resulting triplet excitons may not be collected by an emitting dye, or fluor present in the plastic scintillator, which is typically a singlet emitter, thereby diminishing the luminescence light yield of the plastic scintillator. It has also been found in some approaches that adding an effective amount of a high Z component in a plastic scintillator to enhance photoelectric absorption has resulted in a yellowing or browning of the plastic scintillator, thus yielding insufficient optical clarity in the scintillating wavelength region.
Moreover, as plastic scintillators are typically comprised of mostly hydrocarbons, such plastic scintillators are unable to detect the large fraction of low-energy (thermal neutrons) that do not generate enough light in elastic scatter interactions. In addition to neutron detectors, neutron imagers based on lens-coupled scintillator screens typically use standard plastic scintillators. For these and other neutron detection systems employed to register neutrons throughout the energy range from multiple MeV down to thermal, employ only proton recoils, which produce relatively low light yields, rendering this approach of minimal value once the neutrons have insufficient energy to produce a detectable signal. For these applications, introduction of a neutron capture agent, such as Lithium, provides a strong signal for thermal neutron detection.
Present techniques for detection of thermal neutrons are typically based on 3He detectors. However, due to the imminent shortage of 3He, other neutron detection technologies utilizing 10B- and 6Li-loaded scintillating materials have been considered as possible replacements for 3He detectors. The neutron detection properties of 10B- and 6Li-containing scintillators are based on known capture reactions:10B+n°=7Li+α+2.79 MeV6Li+n°=3He+α+4.78 MeV.
Current scintillators utilizing the thermal neutron detection capabilities of 10B generally use boron in a gaseous form, as BF3, or in a solid form consisting of pure boron or a compound mixture (e.g. boron carbide) present in a matrix. Moreover, current scintillators utilizing the thermal neutron detection capabilities of 6Li include inorganic single crystals (e.g. LiI, LiF, Li-aluminate, Cs2LiYCl6:Ce (CLYC), 6Li-lanthanide borate, etc.), 6Li-loaded glass scintillators, and composite materials including dispersions of nano- or micro-particles of different Li-containing compounds (e.g. nano- or microscale Li-containing crystals) in liquid or plastic matrices.
However, there are several disadvantages associated with existing 10B and 6Li based detectors. For example, single crystal detectors are limited by the size of the crystals that can be grown and the high cost of doing so. Additionally, difficulties that may arise from use of 6Li-loaded and/or 10B-loaded glass scintillators include long decay times and high sensitivity to gamma rays due to the presence of relatively high-Z constituents in their compositions.
Drawbacks with composite materials comprising dispersions of nano- or micro-particles of scintillating compounds in polymer and/or liquid matrices includes elevated levels of light scatter due to inhomogeneous composition and structure, as well as limitations in effective neutron detecting due to insufficient optical clarity in the scintillating wavelength region. Moreover, the lack of solubility of highly polar Li-containing compounds in non-polar aromatic matrices needed for efficient cintillation is another obstacle preventing the use of 6Li-loaded plastic scintillators. While 10B-containing compounds, such as ortho- and meta-carboranes, may be easily introduced into the composition of plastic scintillators, the fabrication of such detectors may be cost prohibitive due to the high expense associated with isotopical enrichment of carboranes from ˜19% of 10B in natural abundance to over 90% needed for the neutron capture probability.
An additional detector design allowing both gamma spectroscopy and neutron detection employs a Bismuth loaded plastic with a surface coating of a 6Li-loaded phosphor. It should be noted that the large volume of the plastic scintillator with high concentration of protons is an effective neutron thermalization medium. This type of detector, known as a “phoswitch,” employs PSD to distinguish between events exhibiting the decay time of the plastic scintillator (from gamma interactions), typically ˜1 ns, and neutron capture events in the 6Li-loaded phosphor, which exhibit the characteristic decay time of the phosphor, typically in the 100-1000 ns range. In this way, a single, low-cost, large-volume plastic scintillator detector, utilizing a single Photomultiplier Tube (PMT) for readout, can provide both gamma spectroscopy and neutron detection.